A printing device for ejecting droplets of an electrically conductive fluid such as a molten metal is known. In such a printing device, a Lorenz force is generated in the molten metal due to which a droplet is expelled through a nozzle of the printing device. Such a device may be used for ejecting droplets of a fluid having a high temperature, such as a molten metal or a molten semiconductor.
Direct printing of molten metals may be employed for printing electronic circuitry, for example. In such an application it is essential that all droplets are actually printed accurately as otherwise the electronic circuitry may not function due to an interruption in the electronic connections as a result of a missing droplet, for example. Therefore, it is desirable to at least monitor that all droplets are actually generated.
Monitoring may be performed by applying a scanning unit that images the printed dots. Such a monitoring system may however be insufficient for correcting any missing or misdirected dots. Therefore, it is desirable to have a system that is able to monitor continuously whether the nozzles of a printing system function properly.
EP 1013453 B1 describes a method for monitoring whether a nozzle operates properly. A printing apparatus is described, which is adapted to eject ink droplets from a nozzle, comprising at least one nozzle provided with an electromechanical transducer, a drive circuit provided with a pulse generator to energise the transducer, a measuring circuit for measuring an electrical signal generated by the transducer in response to energisation, and means to break the circuits in such manner that the drive circuit is open if the measuring circuit is closed. An example of an electromechanical transducer is a piezo element. Such an element changes shape when a voltage is applied to it. This leads to the ejection of a droplet of ink. After a droplet has been ejected, there will be a residual pressure in the nozzle. Because of this pressure, the piezo element will generate an electric signal, which may be measured, as a current or a voltage. The electric signal is dependent of the state of the nozzle. For example, if there is an air bubble present in the nozzle, the damping (residual pressure as a function of time) will be different than in the case of a completely full nozzle. In the process, the piezo element is switched into a measuring circuit after expiry of the actuation so that the said electric signal can be measured. By comparison with a reference signal, i.e. the signal generated by the transducer of a nozzle defined as normal, it is then possible to determine whether the nozzle is in good condition or whether there is a problem which may influence the print quality. If a deviation is found, a repair action may be carried out, for example flushing with ink. In brief, using the known method, it is possible to check continuously for proper operation of a nozzle and repair if a problem occurs. In this way, a permanently good print quality can be achieved.
However, to carry out this method of monitoring the state of a nozzle, a piezo element is necessary. A piezo element cannot operate and function properly under certain conditions, such as at a high temperature. As a consequence, in a printing apparatus adapted to jet fluid at a high temperature, such as a printing apparatus adapted to jet molten metals, a piezo element cannot be used. Therefore, the solution proposed in the cited document, is not suitable for use in this field of technology.
It is an object of the invention to provide a method and a system for monitoring a jetting performance of a system being able to jet conductive fluids.